(1) Field of the Invention
The present invention relates to antireflective coating structures, and particularly relates to an antireflective coating structure for a photonic crystal and relates to a method for forming such an antireflective coating structure.
(2) Description of the Related Art
Recently, photonic crystals have been attracting much attention because they have specific optical dispersion characteristics and a function of confining light in a minute space having submicron dimensions.
A photonic crystal is defined as a structure in which two or more media having different refractive indexes are periodically arranged at intervals of a submicron length, which corresponds to a light wavelength, in a two or three-dimensional pattern. Another structure in which such media are arranged in a one-dimensional manner includes a dielectric multilayer film. A crystal having such a one-dimensional structure is herein included in the photonic crystal because of the following reasons: such a crystal has specific optical characteristics depending on the periodicity of arrangement and useful information applicable to the photonic crystal having such a two or three-dimensional structure can be obtained from the theoretical analysis of the one-dimensional structure crystal.
The photonic crystal includes a two-dimensional triangular lattice photonic crystal. This two-dimensional triangular lattice photonic crystal has a host medium and atomic media, having a cylindrical shape, disposed in the host medium in a two-dimensional triangular lattice pattern. The first Brillouin zone corresponding to the two-dimensional triangular lattice has a regular hexagonal shape. Each vertex of the hexagon corresponds to the K point, the midpoint of each side corresponds to the M point, and the center of the hexagon corresponds to the Γ point. It is known that such a photonic crystal has an energy band (photonic band) with respect to a light wave propagated therein.
The energy band of the photonic crystal is obtained by calculation. In the energy band, there is an energy region in which light is not propagated and this energy region is called a photonic band gap (PBG). In the energy region except for PBG, light is propagated in the photonic crystal and there is a portion in which the wavelength dispersion is significant. An exemplary device using this wavelength dispersion characteristic includes a spectral-separation circuit disclosed in Japanese Unexamined Patent Publication (JP-A) No. 11-271541 (hereinafter referred to as a first related art), and a wavelength-dispersion compensator disclosed in Japanese Unexamined Patent Publication (JP-A) No. 2000-121987 (hereinafter referred to as a second related art).
In order to commercialize such devices disclosed in the first and second related arts, light must enter the photonic crystal for the devices with high efficiency through a medium, which is air in general, disposed on the photonic crystal. Therefore, the photonic crystal needs to have a non-reflective coating thereon.
In a structure in which two different homogeneous media having different refractive indexes are in contact with each other, an antireflective coating is used in general in order to reduce the refractive index of the interface to zero. This antireflective coating is called a λ/4 film in some cases because the thickness is equal to about one fourth of the wavelength λ of the incident light.
However, the reflectivity of the photonic crystal cannot be simply defined in the same manner as that for homogeneous media and the region of the photonic crystal cannot also be defined clearly. Therefore, the above technique in which the antireflective coating is used for the homogeneous medium cannot be applied to the photonic crystal.
Instead of the photonic crystal having the antireflective coating, a photonic crystal structure having protrusions is disclosed in the preprint of the 62nd annual meeting of the Japan Society of Applied Physics, page 1065. This technique is hereinafter referred to as a third related art. This photonic crystal structure has a two-dimensional triangular lattice photonic crystal region in which cylindrical cavities are arranged in an Si host medium and protrusions disposed at the boundary between the photonic crystal region and the homogeneous Si region, wherein the protrusions are cavities having a shape of a combination of a triangular prism and a semicircular pillar. This photonic crystal is improved in transmissivity, by about 15 dB, at the boundary between the photonic crystal region and the Si portion, as compared with other photonic crystals having no protrusion.
In the third related art, the protrusions need to be cavities having a length of 1 μm or less and an angle of 10–20°, which is very small. When such protrusions are actually formed by a dry etching process, there is a risk that the sharp edge of each protrusion is rounded depending on the processing accuracy. Thus, it is difficult to precisely form the protrusions according to the design.
As described above, the above conventional techniques in which the antireflective coating is used for the homogeneous medium cannot be directly applied to the photonic crystal. Furthermore, it is difficult to make practical use of the above technique using the photonic crystal structure having the protrusions for preventing the reflection.